An advantage of this type of hydrophone is that it does not have a resonant mode with a high electromechanical coupling coefficient in or near its useful operating band, thereby avoiding the problem of dispersion of the hydrophonic sensitivity Sh of the sensor and the risk of saturation in the presence of a scrambler if the resonance-induced overvoltage is too high. In the sensor, bending modes exist in the useful band but they exhibit very low mechanical coupling due to the twin layer assembly, i.e. an assembly in which the piezoelectric layer is sandwiched between two electrodes. Furthermore, the transducer is highly resistant to hydrostatic pressure; indeed, since the loading is hydrostatic in nature (substantially equal in three directions), the Von Mises stress in the materials remains low. This absence of resonant modes with a high coupling coefficient in or near the operating frequency band of the device makes it possible to produce sensors according to the invention of large size without necessarily having to group multiple elementary sensors together, generally four being used to obtain the required surface filtering effect or to obtain good acceleration compensation. This makes it possible to keep the costs of the sensor down by obviating the need for additional wiring procedures and additional parts for the relative positioning of the elementary sensors.
The current trend is to increase the frequencies of the acoustic waves emitted by the objects sought by passive sonar. However, since the various forms of self-noise (noise radiated by the hydrophone-bearing structure or support, noise due to the direct flow of water against the hydrophone) decrease rapidly with frequency, the noise level at the beamforming output becomes limited by the electrical noise of the hydrophone channel, which makes it necessary to produce electroacoustic transducers having the lowest possible electrical noise. Furthermore, the increasingly common use of devices for isolating the array with respect to mechanical noise (such as sound baffling devices), or of adaptive processing algorithms, also has the effect of further decreasing the mechanical noise level at the beamforming output. Additionally, the decrease in self-noise is even more substantial when the support structure is coated with a masking and/or anechoic material, the aim of which is to improve the acoustic quieting of the support structure (noise radiated by the support structure) and stealth (emission of target echoes with respect to active sonar). Specifically, this type of coating decreases the mechanical noise level at the beamforming output further still, which noise is the main contributor to self-noise when the hydrophone is fixed or moving at low speed. The suppression of the signal response gain due to reflection off the support makes the need to decrease electrical noise, expressed in terms of equivalent isotropic noise level, all the more critical.
Specifically, the electrical noise, in terms of equivalent isotropic noise level, of a hydrophone is given by
            N      FV      A        =                            N          FV                ⁢                  HG          A                            S        FV              ,where NFV is the electrical noise level at the beamforming output, HGA is the gain of the hydrophone in terms of isotropic ambient noise, i.e. directivity factor (DF), and SFV is the response level in terms of array signal. For an identical level of electrical noise (in V2/Hz), the decrease in the level of the signal response SFV results in an increase in the level of electrical noise NFV expressed in terms of equivalent isotropic noise (in Pa2/Hz), the directivity factor being little affected by the coating of the support.
It is therefore essential to propose a hydrophone that meets stricter requirements in terms of electrical noise of the hydrophonic channel. Stated otherwise, it is essential to propose a hydrophone having low electrical noise in order to facilitate operation at higher frequencies. The electrical noise of the hydrophonic channel is chiefly determined by the electrical noise of the hydrophone (in terms of equivalent isotropic noise) given by the inverse of Sh2Ch/tan(δ), where Sh is the hydrophonic sensitivity expressed in terms of V/μPa, Ch is the capacitance expressed in farads, and tan(δ) is the dielectric losses. However, the main problem of the multilayer elementary sensor with uniform piezoelectric plates resides in the fact that the dielectric losses of the PVDF material increase substantially with frequency, resulting in an increase in the electrical noise of the elementary sensor.
In order to obtain an acceptable level of electrical noise, the usual approach consists in maximizing the volume of active material. This may be achieved by either maximizing the area of the sensor (increasing the capacitance Ch), or by increasing its thickness through the addition of additional layers of PVDF (increasing Sh). However, this solution does not allow the requirement in terms of angular coverage of the array at the highest frequencies to be met, since this requirement involves a decrease in the area of the elementary sensor. Furthermore, it does not allow a constraint to be met on the integration of the sensor within a given volume of thickness, which does not allow the thickness of the PVDF material to be increased indefinitely for the purpose of improving the sensitivity or the capacitance of the elementary sensor. This volume-related constraint is in particular due to the passband of the hydrophonic sensitivity on the support structure.
In order to improve the electrical noise of the multilayer sensor without increasing the volume of piezoelectric material, another approach consists in using materials with a better figure of merit (product of Sh2Ch/tan(δ) per unit volume). These materials are chiefly piezoelectric composites, or piezocomposites, for which there is a specific classification scheme. The classification of piezoelectric composites is based on phase topology rather than on their geometry. For example, an m-n piezocomposite, where m∈{0, 1, 2, 3} and n∈{0, 1, 2, 3}, refers to a two-phase composite the first phase (phase 1) of which is connected along m directions and the second phase (phase 2) of which is connected along n directions. The connectivity solely determines the number of directions along which the various domains of one and the same phase are connected. The shape and size of the phases may be arbitrary. Typically, only one phase is piezoelectric, the other being a non-piezoelectric solid or fluid material such as a polymer or air. Phase 1 is generally used to refer to the piezoelectric material and phase 2 to the non-piezoelectric material. For a piezocomposite of given connectivity, multiple configurations are possible along the directions of connectivity with respect to the direction of polarization of the piezomaterial.
A first group of higher performance materials consists of piezoelectric polymers, such as for example the copolymer P(VDF-TrFE) or porous PVDF. The copolymer P(VDF-TrFE) is a single-phase piezoelectric polymer which is around 50% more sensitive in hydrostatic mode and in blocked mode than PVDF, to the detriment of permittivity which is around 30% lower. Porous PVDF is a piezoelectric composite that consists of a PVDF matrix with fully encapsulated microscopic air inclusions. Porous PVDF is therefore a piezocomposite with 3-0 connectivity. The addition of microfissures to the PVDF makes it possible to double the sensitivity of the material in hydrostatic mode, to the detriment of permittivity, which is around 20% lower. However, no additional improvement is observed in blocked mode, mainly due to the high Poisson's ratio. These materials make it possible to decrease the electrical noise of the sensor by a few dB for a given volume, but with a corresponding increase in the supply cost. Furthermore, the static pressure resistance of porous PVDF is limited and its properties are irreversibly degraded above 70 bar.
A second group of higher performance materials consists of 1-3 composite materials. Columns of piezoelectric material are positioned in parallel to one another along a direction of polarization that is perpendicular to the plane of the layer, and are spaced apart from one another. The columns are embedded within a matrix of polymer material that exhibits low stiffness in comparison to that of the piezoelectric material. The effective mechanical stiffnesses of the composite in the vertical and lateral directions are substantially lower than those of the initial piezoelectric material. In the presence of rigid electrodes, this results in the vertical stress in the piezoelectric columns being amplified, the lateral blocking effect of the electrodes being enhanced, and the effective Poisson's ratio being decreased. Unfortunately, in this type of composite, the filler material is exposed to static pressure on the lateral faces of the plate. The choice of filler material therefore results from a trade-off between performance and resistance to submersion, i.e. between low stiffness and high yield strength. Relatively rigid and resistant polymers are therefore typically used, which limits the potential sensitivity gain and restricts the field of application to rigid piezoelectric materials (ceramics or piezoelectric single crystals). 1-3 composites allow the electrical noise of the sensor to be substantially decreased but provide mediocre sensitivity at a given thickness, which tends to have a negative impact on the electrical noise of the entire channel (then chiefly dominated by the noise of the pre-amplification chain). In order to compensate for this phenomenon, a portion of the area of the sensor must be exchanged for thickness in order to improve sensitivity, to the detriment of the ease of integration of the sensor. Piezoelectric composites with 2-2, 3-1 or 3-2 connectivity can also be used. However, the configurations explored until now suffer from the same limitations, due to the fact that the filler material is still in communication with the exterior and subjected to static pressure.